This invention relates to a rotor electrode for a distributor for use in an internal combustion engine and, more particularly, to a distributor rotor electrode on which a high-resistive layer for suppressing noise is provided on an electrode surface.
FIGS. 17 to 19 illustrate one example of a conventional distributor 1 disclosed in U.S. Pat. No. 4,007,342 to which a distributor rotor electrode 2 of the present invention may be installed. The distributor 1 comprises a rotary shaft 3 to which the rotor electrode 2 is connected for rotation therewith and a plurality of stationary electrodes 4 disposed around the rotary shaft 3.
The rotor electrode 2 comprises an electrode arm 5 securely connected at one or inner end to the rotary shaft 3 of the distributor 1 for rotation therewith in a plane perpendicular to the rotary shaft 3. The rotor electrode 2 also comprises an electrode head 6 integrally connected to the other or outer end of the electrode arm 5. The electrode head 6 comprises a pair of substantially flat surfaces 7 substantially parallel to the plane of rotation of the electrode arm 5 and a substantially arcuated, convexed discharge surface 8 substantially perpendicular to the plane of rotation of the electrode arm 5. The discharge surface 8 is adapted to face toward the stationary electrodes 4 of the distributor 1 to successively define a discharge gap 9 between the discharge surface 8 and the successive stationary electrodes 4 as the rotor electrode 2 rotates. As best seen in FIG. 19, the discharge surface 8 as well as the flat parallel surfaces 7 are coated with an electrically high-resistance layer 10 in order to suppress the noise-generating radiation. The high-resistance layer 10 is made of an oxide such as silicon dioxide, copper oxide, aluminum oxide and Invar oxide.
With this distributor rotor electrode 2, the discharge surface of the base material of the rotor electrode head 6 is completely coated with the electrically high-resistance layer 10, the electric discharge between the rotor electrode 2 and the stationary electrodes 4 cannot take place between the outer circumferential surface of the high-resistance layer 10 and the stationary electrode 4. Rather, this discharge takes place due to an insulating breakdown of a portion of the high-resistance layer 10 by a main discharge which is induced by a partial discharge generated at an interface between the discharge surface of the electrode head 6 and the inner surface of the high-resistance layer 10 at which the layer 10 is attached to the electrode head 6. Accordingly, initial main discharging is difficult to occur and unstable, so that an effective and reliable noise suppressing effect is difficult to obtain.
The high-resistance layer 10 is formed by a surface treatment process having at least one of the following surface treatment methods alone or in combination:
(1) flame spraying a high-resistance substance to the rotor electrode; PA1 (2) flame spraying a metal exhibiting a high electric resistance when oxidized, and oxidizing the flame-sprayed metal; and PA1 (3) oxidizing a metal exhibiting a high electric resistance when oxidized, and flame spraying the oxidized metal.
According to this process, however, many defects in the form of voids appear not only in an interface between the electrode and the flame-sprayed layer, but also in the flame-sprayed layer itself, so that the bonding strength of the layer with respect to the rotor electrode is low and that slight changes in the flame-spraying conditions greatly affect the noise-suppressing effect of the flame-sprayed layer.
Another example of a conventional distributor rotor electrode disclosed in Japanese Patent Publication No. 61-38351 has at least one layer of Si varnish or an SiO.sub.2 sheet including an organic binder bonded by an organic material to a thin rotor electrode. In this arrangement, a relatively thin electrode head is exposed to the discharge gap at its discharge surface in order to provide a narrow surface for an improved noise suppressing effect. With this distributor rotor electrode, however, since the organic material used to bond Si varnish or SiO.sub.2 sheet to the electrode are exposed to electric arcs and the O.sub.3 gas or NO.sub.x gas generated upon arcing in the discharge gap, they are relatively quickly deteriorated and the Si varnish or SiO.sub.2 sheet can be relatively easily separated, thereby shortening the lifetime of the rotor electrode.